1. Field of the Invention
The invention is generally related to the area of optical communications. In particular, the invention is related to a motor driven variable optical attenuator with IR sensor closed-loop control.
2. The Background of Related Art
The future communication networks demand ever increasing bandwidths and flexibility to different communication protocols. DWDM (Dense Wavelength Division Multiplexing) is one of the key technologies for such optical fiber communication networks. DWDM employs multiple wavelengths or channels in a single fiber to transmit in parallel different communication protocols and bit rates. Transmitting several channels in a single optical fiber at different wavelengths can multi-fold expand the transmission capacity of the existing optical transmission systems, and facilitating many functions in optical networks.
In general, the channel signals come from different sources and may have transmitted over different mediums, resulting in different power levels. Without equalizing the power levels of the channel signals that are to be combined or multiplexed, some channels in a multiplexed signal may be distorted as a result of various stages of processing the multiplexed signal. On the other hand, many optical devices or systems would not function optimally when incoming signals are beyond a predetermined signal level range. In fact, the power of the incoming signals shall not be too low, neither too high. To ensure that all optical devices or systems receive proper levels of optical signals, attenuation devices are frequently used to adjust the optical signals before they reach an optical device.
Electrically tunable optical devices are widely deployed in various fiber optical communication systems. An important feature in current tunable optical products is to allow a device to perform optically at a docking or reset position after its power supply has been cut off. For example, for an electrically controlled variable optical attenuator (EVOA), such a desired docking or resetting function can be a “dark” or “bright” attenuation state that satisfies the following requirements, respectively: when an applied external electric signal is turned off, the EVOA output is “dark” or “bright” thus exhibits a high or low attenuation to the input signal, respectively. Here the high or low attenuation typically has its quantifiable specifications and could be, for example, >30 dB or <1 dB, respectively. As another example, for an optical switch, this docking or resetting feature is often referred to as a non-latching function, meaning regardless of the switching state before the power off, the switch should return to a specific and predetermined switching state after the power is turned off accidentally or intentionally. Still as another example, in an tunable optical filter, this docking or resetting feature defines a fixed filter output spectrum location and shape within its tunability range.
Some optical devices allow such a reset function to be implemented in a straightforward way. For example, for a tunable optical device based on a micro-electro-mechanical-system (MEMS) platform, a designer may take advantage of various mechanical material and structure properties of the MEMS fabrication to design in a spring-loaded micro hinge to reset an optical actuator that performs the tuning. For this type of device, once its power is cut off, the MEMS micro hinge springs back the actuator to a reset state, thus implementing a predetermined reset function.
A prior art MEMS EVOA is sketched in FIG. 1 where only basic elements are illustrated. The actuator (not shown) of the EVOA could be an electrostatic mirror that responds to a driving voltage signal by tilting its angular position to deflect the input light to miss a part of the entire aperture of the output collimator, thereby causing various light attenuations. In the MEMS EVOA design of a “dark” reset feature, a micro spring-loaded hinge is used so that the mirror's relaxed position is an off-angle position where an optical alignment will not allow the mirror reflected light to enter the EVOA's output port. Only when certain control signals are applied to overcome the spring force, can the VOA be tuned to have specific attenuation as the entire collimated beam is aimed at the output aperture. On the other hand, for a “bright” reset feature implementation of a MEMS EVOA, one can choose the spring relaxed position to be the one with the right alignment so that all the light can be reflected by the mirror into the output collimator aperture.
Many existing tunable optical devices, however, have no such a feature that can be easily implemented with a micro spring-loaded hinge, but they offer other advantages that a MEMS platform does not perform well, such as good repeatability, good device tuning resolution and robust performance against harsh environment where the so-called temperature-dependent loss (TDL) is a main specification to differentiate. Thus, in order for these non-MEMS based technologies to be implemented with the reset features, other methods must be incorporated and integrated.
In the current invention, we disclose techniques to artificially enforce a reset function triggered by a power cut-off and apply it to various non-mechanical spring-based actuators used for various tunable optical devices. In particular, the current invention can be effective for two classes of applications where 1) either a detent force such as a magnetic force can be employed to help reset or 2) no such detent force of any kind exists but such a reset is still in order.